Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Zoology and Genetics


Ecology and Evolutionary Biology

First Advisor

Carol M. Vleck


Life history theory predicts, and empirical studies indicate, that egg size can be an important determinant of fitness for a variety of species. The premise of these studies is that energy for reproduction is limited, and as parents increase energy investments in individual offspring, those offspring should accrue fitness benefits. The work presented in this dissertation addresses how energetic and hormonal resources vary with egg size and the consequences of these investments for offspring growth and survival in a free-living population of American coots (Fulica americana). Eggs vary in size at three levels: at the population level, among females and among eggs within females. Across all egg sizes, large eggs contain absolutely more yolk, albumen, shell and water than small eggs, but these components comprise a similar proportion of egg mass regardless of size. Hatchlings from large eggs have proportionately more yolk and body fat, but proportionately less tissue and water than hatchlings from small eggs. Results from a fostering experiment indicate that differences in absolute egg size do not enhance survival or growth in the first 2--3 weeks after hatching. Most variation in egg size within the population is due to differences among females. However, within clutches, egg size varies predictably with laying sequence. The first egg and later-laid eggs are smaller than eggs laid in the middle of the clutch. This pattern of variation in egg size is related to offspring survival, and weakly to growth. The largest eggs in a clutch have higher survival and growth than the smallest eggs in a clutch. These results suggest that females allocate resources unequally among eggs and these resources enhance offspring fitness. This hypothesis is corroborated by the concordance of intraclutch patterns of yolk-androgen levels with intraclutch patterns of egg-size variation. Maternally-derived androgens in yolks are associated with higher growth and social dominance in other birds and may be functionally linked to the relative egg-size effect on offspring performance in coots. These patterns of maternal effects explain a statistically significant portion of the variation in offspring growth and survival, however, the environmental factors have a much greater impact.



Digital Repository @ Iowa State University,

Copyright Owner

Wendy Louise Reed



Proquest ID


File Format


File Size

164 pages